Conventionally, as this kind of induction heating apparatus, for example, the apparatus described in Japanese Laid-open Patent Application No. Sho 61-16491 was available. FIG. 9 is a view showing the equivalent circuit of the induction heating coil and its peripheral portion in this kind of conventional induction heating apparatus.
The configuration of the conventional induction heating apparatus will be elucidated below using FIG. 9. In FIG. 9, numeral 1 designates a top plate; an induction heating coil 2 is provided below the top plate 1; and a matter 3 to be heated is placed thereon. Numeral 4 designates an electrostatic shield applied to the bottom face of the top plate 1 and electrically connected to the low-potential portion of an inverter circuit (not shown) for driving the induction heating coil 2 via the electrode 4a of the electrostatic shield 4. As the equivalent circuit of the peripheral portion, the equivalent capacitance C1 between the induction heating coil 2 and the electrostatic shield 4, the equivalent capacitance C2 between the matter 3 to be heated and the electrostatic shield 4, the equivalent resistance R1 of a human body at the time when the human body makes contact with the matter 3 to be heated, and the resistance R2 of the electrostatic shield 4 are shown.
In this configuration, when the matter 3 to be heated has low magnetic permeability and is a pan made of aluminum, copper, etc. having a low resistance, the frequency of the current flowing through the induction heating coil 2 is high in comparison with the case when the matter 3 to be heated is an iron pan, that is, a matter having high magnetic permeability and relatively large resistivity and being liable to generate Joule heat; hence, the peak voltage applied to the induction heating coil 2 becomes 1 kV or more.
In the case that the electrostatic shield 4 exists and is electrically connected to the low-potential portion as described above, the potential difference between the matter 3 to be heated and the electrostatic shield 4 becomes small, whereby a leak current at the time when a human body makes contact with the matter 3 to be heated is decreased significantly. Hence, safety is ensured even if the human body makes contact with the matter 3 to be heated.
When the electrostatic shield 4 is electrically connected to the low-potential portion of the inverter circuit for driving the induction heating coil 2, a method wherein one end of a lead wire serving as the connection path thereof is connected to the electrode 4a of the electrostatic shield 4 applied to the top plate 1 by soldering or by contacting an elastic member, such as a spring, to which the one end of the lead wire is connected, and the other end of the lead wire is connected to the low-potential portion of the inverter circuit has been used generally.
However, in the case of the above-mentioned conventional configuration, the strength of the connection between the electrode 4a and the lead wire or the stability and reliability of the connection are insufficient; for example, because of some reasons, such as the occurrence of a tension force during a production process, reduction in the strength of solder owing to the heat from the matter 3 to be heated during cooking and the vibration or drop impact of the apparatus, the lead wire may be disconnected from the electrode 4a, the spring terminal may be oxidized, the contact part between the electrostatic shield and the spring terminal is separated owing to vibration or the like, whereby there is a fear of causing a problem of increasing the contact resistance and preventing the function of the electrostatic shield 4 from performing sufficiently.
In order to solve the above-mentioned conventional problem, the present invention is intended to provide an induction heating apparatus capable of ensuring the electrical connection between the electrostatic shield and the low-potential portion of the inverter circuit and allowing the function of the electrostatic shield to perform sufficiently at all times.